Community-Acquired Pneumonia

Posted by • October 24th, 2014

Community-acquired pneumonia is a commonly diagnosed illness in which no causative organism is identified in half the cases. Application of molecular diagnostic techniques has the potential to lead to more targeted therapy in the face of increasing antibiotic resistance. A new review article looks at this topic.

Community-acquired pneumonia (CAP) is a syndrome in which acute infection of the lungs develops in persons who have not been hospitalized recently and have not had regular exposure to the health care system.

Clinical Pearls

What are the most common causes of CAP?

Although pneumococcus remains the most commonly identified cause of CAP, the frequency with which it is implicated has declined, and it is now detected in only about 10 to 15% of inpatient cases in the United States. Other bacteria that cause CAP include Haemophilus influenzae, Staphylococcus aureus, Moraxella catarrhalis, Pseudomonas aeruginosa, and other gram-negative bacilli. Patients with chronic obstructive pulmonary disease (COPD) are at increased risk for CAP caused by H. influenzae and Mor. catarrhalis. P. aeruginosa and other gram-negative bacilli also cause CAP in persons who have COPD or bronchiectasis, especially in those taking glucocorticoids. There is a wide variation in the reported incidence of CAP caused by Mycoplasma pneumoniae and Chlamydophila pneumoniae (so-called atypical bacterial causes of CAP), depending in part on the diagnostic techniques that are used. During influenza outbreaks, the circulating influenza virus becomes the principal cause of CAP that is serious enough to require hospitalization, with secondary bacterial infection as a major contributor.

Table 1. Infectious and Noninfectious Causes of a Syndrome Consistent with Community-Acquired Pneumonia (CAP) Leading to Hospital Admission.

What evaluation do the authors recommend to determine the cause of community-acquired pneumonia in a hospitalized patient?

In hospitalized patients with CAP, the authors favor obtaining Gram’s staining and culture of sputum, blood cultures, testing for legionella and pneumococcal urinary antigens, and multiplex PCR assays for Myc. pneumoniae, Chl. pneumoniae, and respiratory viruses, as well as other testing as indicated in patients with specific risk factors or exposures. A low serum procalcitonin concentration (<0.1 microg per liter) can help to support a decision to withhold or discontinue antibiotics. Results on Gram’s staining and culture of sputum are positive in more than 80% of cases of pneumococcal pneumonia when a good-quality specimen (>10 inflammatory cells per epithelial cell) can be obtained before, or within 6 to 12 hours after, the initiation of antibiotics. Blood cultures are positive in about 20 to 25% of inpatients with pneumococcal pneumonia but in fewer cases of pneumonia caused by H. influenzae or P. aeruginosa and only rarely in cases caused by Mor. catarrhalis.

Morning Report Questions

Q: What are the guidelines for treating community-acquired pneumonia in outpatients and inpatients?

A: For outpatients without coexisting illnesses or recent use of antimicrobial agents, IDSA/ATS [Infectious Diseases Society of America and the American Thoracic Society] guidelines recommend the administration of a macrolide (provided that <25% of pneumococci in the community have high-level macrolide resistance) or doxycycline. For outpatients with coexisting illnesses or recent use of antimicrobial agents, the guidelines recommend the use of levofloxacin or moxifloxacin alone or a beta-lactam (e.g., amoxicillin-clavulanate) plus a macrolide. The authors argue, however, that a beta-lactam may be favored as empirical therapy for CAP in outpatients, since most clinicians do not know the level of pneumococcal resistance in their communities, and Str. pneumoniae is more susceptible to penicillins than to macrolides or doxycycline. Even though the prevalence of Str. pneumoniae as a cause of CAP has decreased, they raise concern about treating a patient with a macrolide or doxycycline to which 15 to 30% of strains of Str. pneumoniae are resistant. For patients with CAP who require hospitalization and in whom no cause of infection is immediately apparent, IDSA/ATS guidelines recommend empirical therapy with either a beta-lactam plus a macrolide or a quinolone alone.

Q: What is the appropriate duration of antibiotic therapy for community-acquired pneumonia?

A: Early in the antibiotic era, pneumonia was treated for about 5 days; the standard duration of treatment later evolved to 5 to 7 days. A meta-analysis of studies comparing treatment durations of 7 days or less with durations of 8 days or more showed no differences in outcomes, and prospective studies have shown that 5 days of therapy are as effective as 10 days and 3 days are as effective as 8. Nevertheless, practitioners have gradually increased the duration of treatment for CAP to 10 to 14 days. The authors argue that a responsible approach to balancing antibiotic stewardship with concern about insufficient antibiotic therapy would be to limit treatment to 5 to 7 days, especially in outpatients or in inpatients who have a prompt response to therapy. Pneumonia that is caused by Staph. aureus or gram-negative bacilli tends to be destructive, and concern that small abscesses may be present has led clinicians to use more prolonged therapy, depending on the presence or absence of coexisting illnesses and the response to therapy.

One Response to “Community-Acquired Pneumonia”

  1. Dr DK Jha says:

    Good approach and in children for outpatients still amoxicillin remains the drug of choice. For children more than 5 years of age macrolides are drug of choice. In inpatients for children third generation cephalosporins are the first choice. Thanks.